Chemotactic adaptation kinetics of individual Escherichia coli cells

Escherichia coli cells are propelled in aqueous environments by a bundle of rotating helical flagella, and swim in a random pattern composed of straight “runs” followed by abrupt "tumbles".  Like many other bacteria, E. coli sense chemicals in their surroundings and alter their swimming pattern in order to move toward favorable environments, a phenomenon referred to as chemotaxis. Though bacterial chemotaxis has been studied for decades, methods of probing the system have so far been limited to either: (i) monitoring populations of free swimming cells, or (ii) longer-term measurements of individual flagellar motors, rather than observing the physiologically-relevant swimming behavior of the individual, multi-flagellated cell. Researchers at the Center for the Physics of Living Cells (CPLC) used a recently-developed optical trapping technique to characterize the swimming behavior of individual bacteria as they respond to sudden changes in the environment (Min et al., PNAS 2012). The ability to monitor the long-term behavior of the individual cell helped reveal fundamental features of the chemotactic response not readily captured by standard assays.

One of the hallmarks of bacterial chemotaxis is the ability of the cell to adapt to a new chemical environment. Applying varying magnitudes of step-up or step-down changes in concentration of chemical attractant, Min et al. were able to follow the adaptation kinetics of individual E. coli for the first time. In contrast to the prevailing picture in which chemotactic adaptation is described as a gradual process, the researchers found that individual E. coli cells adapt abruptly. The typical response of an adapting cell surprisingly consists of a single prolonged run, followed by an immediate return to its pre-stimulus state. Moreover, the degree of abruptness is a function of the chemical stimulus strength. The single-cell assay also allowed quantifying another feature of adaptation kinetics, an overshoot response, and its dependence on chemical stimulus.  This feature had also never been quantified at the single-cell level before now. This work provides new insight on E. coli chemotaxis, and moves the scientific community closer towards end-to-end understanding of this process, from stimulus to motility.